Field of Invention
The invention relates to a method and system for determining the relative location of objects and features in a plurality of scanned images. The invention is particularly directed to medical and dental applications including those that require surgical and prosthetic devices to be designed and manufactured to precise dimensions dictated by the anatomy of individual patients, and still more particularly directed to the problem of registering, as precisely as possible, digitized 3-D scans of the mandible and maxilla of a patient or, equivalently, casts or impressions of same.
Many surgical procedures concern the temporary or permanent insertion, into the soft or bony tissue of a patient, of prosthetic and other artificial devices that are required to fit the anatomy of the patient to a very high degree of precision and accuracy. One such application concerns implant dentistry, in the course of which one or more (usually metallic) implant anchors are surgically placed within the jawbone of a patient, to receive and support prosthetic components designed to simulate and replace one or more natural teeth lost by the patient. It is well known that, to be wholly successful, implant procedures must adhere to very strict placement, orientation and sizing requirements determined by existing bone structure and dentition, whereby the prosthetic components to be fitted onto surgically-placed implant anchors must preferably be designed, shaped and sized specifically to conform to the precise anatomical geometry of the patient, including the location, shape and size of adjoining teeth, and must transition to the precise orientation of the principal axis of the supporting implant anchor with a high degree of accuracy.
In addition, the development of many products and services provided in the fields of orthodontic and restorative dentistry seek to make use of computer aided design (CAD) and computer aided manufacturing (CAM). For example, in dentistry stone or plaster casts made from impressions of the patient's mouth are commonly used to provide the products or services needed, and three dimensional (3-D) scanning of either the patient's dentition or of casts representative of the patient's dentition are used to provide the dental CAD system with data representing the pertinent geometry. For such applications, however, very accurate alignment of the images of the maxilla (or of the upper cast replica) and the mandible (or of the lower cast replica) are needed for dental CAD modeling.
Conventional methods for meeting these rigorous requirements provide for the creation of a model of the patient's jaw and dentition, the making of said model comprising the taking of a so-called “impression” of the patient's dentition, using a malleable substance placed over and around the teeth in the patient's mouth comprising the entire dental arch. Where the placement of implants and restorative components is a factor, typically this impression is taken following the surgical insertion of the implant anchors. Typically, reference components called impression copings are affixed to the external extremity of the inserted implant anchors, and serve to reference the location and angular orientation of the anchors. Subsequently, a model made from a mold based on said impression will incorporate so-called “analog” anchors to model the anchors in the patient's jaw, and prosthetic devices for said anchors will be designed and manufactured based on the geometry of the model created as described.
In actual practice the conventional procedure described above is fraught with numerous difficulties and shortcomings. It has proven impossible for dental practitioners to make dental impressions, and thus models, that are consistently free of dimensional and positional errors; so rigorous are the geometrical requirements involved in such applications that even a sub-millimeter dimensioning error, or a 1 or 2 degree orientation error, will result in prosthetic placements that give rise to unacceptable stresses and conditions.
In recent years efforts have been made to employ image-based modeling techniques to address these well-known problems of conventional implant dentistry procedures. In these efforts, images are taken of the patient's mouth, and a three-dimensional model of the pertinent regions is recreated using so-called three-dimensional image processing techniques and software. The field of photogrammetry, which traces its origins to the decade following the invention of photography in the 1830s, is “the art, science and technology of obtaining reliable information about physical objects and the environment through the processes of recording, measuring, and interpreting photographic images and patterns of electromagnetic radiant energy and other phenomena.” (Manual of Photogrammetry, American Society of Photogrammetry and Remote Sensing, 4th Ed., 1980). Particularly with the advent of computers having fast processing speeds and large memories, and the advent of low-cost digital cameras and other image-capture devices, off-the-shelf three-dimensional image processing software has become readily available that is applicable to a wide variety of virtual modeling applications. Using such software, it has become possible to reconstruct reasonably accurate three-dimensional models of an imaged subject field using available commercial products. However the particular demands for great accuracy, and the physical strictures of imaging the human body, have thus far resulted in the absence, in the field of dentistry, of acceptable three-dimensional imaging techniques. A particular problem is the necessity, for the accurate reconstruction, in the form of a virtual model, of an imaged scene. Typically, an object is imaged from more than one position, thereby providing a more complete three-dimensional model.
U.S. Pat. No. 5,851,115 issued Dec. 22, 1998 to Carlsson, et al, describes a photogrammetric method and system for imaging the mouth, for the purpose of creating a virtual model of the patient's mouth from which dental parts may be designed and made. In the system according to Carlsson et al a specialized camera is employed, comprising a set of mirrors that enable a single exposure to embody stereographic images from two different angles. The system of Carlsson further requires that the relative geometry of the virtual “lenses” created by the mirror system be known precisely. To assist the software in locating and orienting imaged features, Carlsson teaches the use of reference markings, such as circles, applied to flat surfaces within the imaged field.
U.S. Pat. No. 5,857,853 issued Jan. 12, 1999 to van Nifteric et al. also discloses a photogrammetry-based method for capturing the dimensional and orientation data required for the manufacture of dental prosthetic parts used in implant dentistry. In order to obtain the at-least-two views required by the triangulation engine of the photogrammetry software, the method of van Nifteric et al employs either a plurality of cameras having precisely-known relative positions, or a single camera mounted on a swiveling carriage that is movable between separated but accurately defined positions. van Nifteric et al. further teach the use of recognition objects and points, to serve as reference points used by the photogrammetry software in positioning features of the imaged scene within a coordinate frame. van Nifteric et al. thus disclose the use of a bar comprising measuring scale markings, and of two spheres mounted on a pin, as recognition objects.
While the methods disclosed in the Carlsson et al. and van Nefteric et al. patents constitute significant advances, these methods still exhibit several important disadvantages and shortcomings that render them impractical for most implant dentistry practitioners. Both of said methods require the use of highly specialized and accordingly expensive camera equipment, and both require that such camera equipment be precisely aligned, to capture a plurality of images from precisely known relative lens positions. Functionally, both methods are inadequate to image accurately a wide field of view, particularly a wide field of view comprising areas characterized by very low feature definition, a condition typical of the edentulous (tooth-free) jaw and thus quite common in implant dentistry practice. The present invention addresses these shortcomings of the prior art, and it provides a three-dimensional-based virtual modeling method, specifically directed to medical and dental applications, that is remarkably low cost and that provides improved feature reconstruction accuracy particularly in applications that require the use of combined three-dimensional images.
Specifically with respect to the problem of providing dental CAD systems with the relative position of maxilla and mandible, prior art methods have depended on two methods that share a common characteristic: the first method relies on capturing the 3-D image of the facial surface of both the maxilla and the mandible in a single image. Separate, individual scans of the maxilla and mandible are then matched to the common template provided by the image of the facial surface of both maxilla and mandible. The second prior art method relies on capturing the “bite registration”, or impression of occlusal or biting surfaces of both the upper and lower teeth. After the geometry of the mandible has been captured, the bite registration is placed on the surface of the mandible and it also is scanned. The scan of the maxilla is then matched with the image of the matching surface of the bite registration.
Both of the prior art methods described in the preceding paragraph suffer from two fundamental problems. One problem is computational complexity, and the need to minimize even greater computational complexity by means of a good initial guess by a human operator regarding the relative position of the images being matched. A second and still greater difficulty arises where the patient is partially or completely edentulous, and there is lacking the 3-D information necessary to match the scans of maxilla and mandible. Also, it is difficult to obtain the geometry of anterior teeth from a bit registration.
In some prior art, the object is scanned, using any suitable scanning means capable of capturing a cloud of data points representing three dimensional features of the scanned field. Such scanning typically requires the taking of a plurality of overlapping images that collectively span the image field to cover the required. Various methods are typically employed to recreate the entire three dimensional model from these separate scans. One such prior art method uses precise information about the location of the model with respect to the camera to position and orient the multiple images. In addition, commercially available three-dimensional image processing software products also provide tools to combine discrete scans into a single model by matching the overlapping regions of the images. Well-known examples of suitable image-processing software include the Studio software marketed by Raindrop Geomagic, Inc.